The present invention relates to disk storage devices and in particular to balancing a disk pack assembly about a disk spindle axis of rotation.
Disk assemblies usually comprise a plurality of disks with information stored on annular tracks closely packed together to increase data density. The disks are fastened about a hub. The disk assemblies rotate at high speeds to decrease the time it takes to obtain data via a sensing head which is substantially stationary with respect to the rotation of the disks, but shifts radially across the data tracks to sense data on different tracks.
An out of balance condition in the rotating disk assembly causes erratic speed variations with respect to the tracks and the sensor which results in read/write errors, planar and axial vibrations of the disk surfaces which contribute to sensor crashes destroying both the sensor and the disk surface, and decreased bearing life of disk drives.
Prior attempts to solve the above problems involved designing and manufacturing disks and hubs to tight tolerances to attain as close a fit between the inner diameter of the disks and the outer diameter of the hub as possible to center the rotating mass with respect to the axis of rotation of the disk assembly. This still resulted in out of balance conditions because of the inability to attain perfect fits and uniformly balanced disks. To obtain better balance, balance rings were used. By either adding or removing material from the balancing ring as indicated by sophisticated balancing equipment better balance was obtained, but at the expense of further steps following assembly of the disk packs.
U.S. Pat. No. 4,358,803 to Van Der Giessen describes accurately machined inner walls of disc central openings and a centering element which cooperates with at least one of the inner walls to center disks. In U.S. Pat. No. 4,224,648 to Roling, centering was performed using a steel centering ball in the center of a disk pack having a hemispherical surface facing a spindle cup. Centering with respect to the inner walls of a disk does not ensure that the disk is centered with respect to the majority of rotating mass of the disks which lies almost entirely outside the inner wall of the disk. The outer diameter of a disk may not be perfectly concentric with the inner diameter of the disk. Thus, centering with respect to the inner diameter of the disks requires high precision in aligning disks before the fastening of the disks to the hub and neglects some of the major causes of imbalance.
The above methods of centering and fastening disks also lead to particle generation which degrades performance of disk drives. Mating surfaces, such as the machined inner walls of disc central openings and the centering element in U.S. Pat. No. 4,358,803 and the steel ball and spindle cup in U.S. Pat. No. 4,224,648 are designed to slide while subjected to pressures until mating occurs. This sliding produces undesirable particles. Clamping by use of screws is also a notorious source of particulate contamination.